Hot, dry weather over the last two decades already has contributed to the deaths of an unusual number of old-growth pine and fir trees growing in Yosemite and Sequoia National Parks, according to recent research from the U.S. Geological Survey.

In the next decade, climate change also could start interfering with the giant sequoias' ability to sprout new seedlings, said Nathan Stephenson, one of several scientists speaking last Thursday at a government agency symposium on how global warming could affect the Sierra Nevada.

"The first effects of climate change that we're likely to see is that the giant sequoias will have trouble reproducing because their root systems don't work as well when temperatures warm," said Stephenson, a research ecologist with the USGS Western Ecological Research Center. "After that, I wouldn't be surprised if in 30 years we see their death rates go up."

Sequoiadendron giganteum, an inland cousin to the tall California coast redwood, can become 2,900 years old and bulk up to more than 36 feet in diameter, making them among the world's most massive living things.

Stephenson was among a team of tree demographers who monitored the health of pines and firs growing in the two southern Sierra Nevada parks from 1982 to 2004.

As both temperatures and summer droughts increased over that period, he found the trees' normal death rate more than doubled, and stands became more vulnerable to attacks from insects or fungus.

While those species have a faster life cycle than the ancient sequoias, scientists say the mortality rates can help predict what may happen to the massive organisms as temperatures increase as predicted an average of 3 to 10 degrees Fahrenheit statewide by the end of the century.

"We've got a lot of our most cherished species at stake," said Constance Millar, a senior research scientist with the U.S. Forest Service. "Rather than just managing forests for the plants we see growing there today, we're now having to look forward to think about what might thrive there in 100 years."

Native flora and fauna throughout the 400-mile-long Sierra Nevada mountain range are already under stress from a warming climate, and federal land managers have started monitoring wildlands to understand how they're transforming.

Some officials have already started making changes based on what they see on the ground.

Recently, the Forest Service redrew its decades-old maps for where to place fire breaks along the Sierra Nevada, moving suppression efforts down from the ridge lines to lower regions where scientists now believe habitats are at risk from wildfires, Millar said.

One local species troubled by rising temperatures is the mountain-dwelling American pika, or rock-rabbit. The 6-inch-long rodent thrives in cool, mountaintop climates, but at higher temperatures they can overheat and die within hours.

The population has been dwindling and drifting to ever higher elevations, but biologists fear it eventually could run out of mountain.

Still, because it could take years to understand how different animals and plants are influenced by not only rising temperatures, but fires, pollutants, forest management practices and other change agents, park officials said need to proceed cautiously.

"Right now, we're going to focus our efforts on the big icon for the parks, the giant sequoias," said Craig Axtell, superintendent of Sequoia and Kings Canyon National Parks. "But we may find that other problems come up down the road that we don't even know about."